The invention relates to a high-resolution thermopile infrared sensor array having monolithically integrated signal processing and a plurality of parallel signal processing channels for the signals from pixels of a sensor array, and also a digital port for the serial output of the signals of the pixels, wherein the sensor array is situated on one or more sensor chips.
In order to construct a high-resolution thermopile infrared sensor array, the number of individual thermopile elements, i.e. the number of pixels, must increase and the geometric dimensions of the pixels themselves must decrease. The thermopile sensor arrays originally available commercially comprised only a small number of pixels (e.g. 8×8 pixels or 16×16 pixels), wherein the individual pixels were very large (e.g. 150 . . . 300 μm×150 . . . 300 μm). There was thus enough space on the sensor chip (e.g. composed of silicon) to accommodate a few amplifiers or low-pass filters on the chip alongside the thermopile sensor array.
Currently customary thermopile sensor arrays having larger numbers of pixels require a reduction of the dimensions of the pixels to a side length of 100 μm, or even down to 25 μm. However, the smaller pixels resulting from the increasing integration density have the disadvantage that they generate an area-proportionally smaller signal. That means that, given half a pixel size, only one quarter of the signal strength is available for the further processing.
The signal voltages, which are thus ever lower and are usually in the nV range up to a few μV, require ever higher gain factors in order that signals can be processed further without additional noise or other interference influences outside the actual sensor housing. The consequence, however, is an ever lower signal-to-noise ratio.
The signal gain required as a result of the low signal voltages is at least a few 1000, usually even above 10,000, in order to raise the signal voltage to an extent such that it can be passed on to other assemblies and processed further. The high-gain analog amplifiers traditionally used for signal amplification require multi-stage amplifiers with a relatively large area requirement and, moreover, these amplifiers have a considerable current consumption. That means that the power loss and thus at the same time the inherent heating increase, which in turn leads to an increase in the measurement error of the thermopile sensor array overall.
In the meantime, infrared thermopile sensor arrays produced on one chip by means of silicon micromachining have become known in a variety of embodiments. In these sensor arrays, a portion of the signal processing takes place on the chip, but only a small number of preamplifiers and a common multiplexer are present, the latter outputting the signals of all the pixels. The thermopiles have so-called “hot” contacts on an infrared reception area becoming smaller and smaller, and so-called “cold” contacts on a heat sink at the edge of the respective pixel. The signal voltage generated by the thermopile is directly dependent on the achievable temperature difference between the “hot” and “cold” contacts.
By way of example, in the case of a thermopile sensor array in accordance with WO 2006/122529 A1, for each row of the sensor array respectively one preamplifier and one low-pass filter are integrated on the chip. That is insufficient, however, for high-resolution sensor arrays having many rows and columns. By way of example, only 64 preamplifiers and 64 low-pass filters are used in the case of an array of 64×64 pixels. The achievable noise bandwidth would be up to 64 times higher than necessary. Since the noise increases with the root of the noise bandwidth, however, the noise could be reduced by up to 8-fold, or the thermal resolution capability could be improved by up to 8-fold.
However, no measures are specified for a current- and space-saving solution for the signal amplification per signal channel.
Furthermore, JP 2004-170375 A discloses a thermopile sensor array which comprises only a single preamplifier.
DE 103 22 860 B4 describes a circuit arrangement for reading out electronic signals from high-resolution thermal sensors with preamplifiers which are situated upstream of a multiplexer. In order to reduce the considerable power loss of the individual preamplifiers operating in parallel, the latter are cyclically switched off in order to save power.
It is precisely as a result of this measure that the high temperature resolution desired is not achieved, however, because the preamplifiers upstream of the multiplexer can obtain the expected result only if the noise bandwidth of the sampled signal is simultaneously reduced proportionally to the number of parallel amplifier channels. That is not possible, however, if the amplified signal, via the preamplifiers, is not continuously “integrated” by means of a low-pass filter for noise bandwidth limiting, or smoothed e.g. via a low-pass filter.
EP 2 587 234 A1 discloses a thermopile infrared sensor having a circuit for signal processing in which the signals of the individual pixels are forwarded without band limiting, or interposed preamplifiers.
All the solutions above describe thermopile infrared sensor arrays, but without proposing measures for signal processing in conjunction with a higher integration density on the chip. In particular, there are no proposals for reducing the noise bandwidth in conjunction with an extremely small space requirement and an extremely low power loss.
The know solutions have an inadequate thermal resolution capability because only a single or only very few preamplifier channels has/have been integrated on the sensor chip, which leads to a high noise bandwidth and at the same time to a poor signal/noise ratio.
A high integration density requires a reduction of the pixel size and the so-called pixel pitch, i.e. the center-to-center distance between the thermopile pixels, in order thereby to accommodate more pixels on the same chip area. In addition, a high thermal resolution, i.e. a high signal/noise ratio, and a low noise-limited temperature resolution NETD (Noise Equivalent Temperature Difference), are also desirable besides a high geometric resolution.
Owing to the smaller reception area and owing to the resultant small distance between the “hot” and “cold” contacts of the thermopile pixels, a reduction of the pixel size simultaneously also leads to a reduction of the sensor signal emitted by the thermopile pixel, which leads to a lower signal/noise ratio, poorer thermal resolution capability and to a reduced measurement accuracy.
In principle, it is possible to reduce the size of thermopile cells (thermopile pixels) and to integrate ever greater numbers of pixels on the sensor chip. By way of example, 16×16, 32×32, 64×64, 128×128 thermopile pixels or more are realized on one sensor chip. The signal voltages of the individual thermopile pixels would have to be multiplexed by means of m×n addressing and MUX switches, i.e. passed onto a common, serial signal line to one amplifier per array, or via a common serial interface per row or column.
Since the signals still able to be minimally resolved from thermopile pixels having further reduced pixel dimensioning are still in the nV range for many applications, the signals still have to be sufficiently highly amplified and processed further on the chip, such that they cannot be influenced by electrical interference influences outside and within the sensor chip.
Gain factors of typically 10,000 or more are necessary in the case of the known solutions in order to amplify the sensor signals of such small thermopile pixels to a few mV, in order that the sensor signals downstream of the MUX (multiplexer) can be output from the sensor chip on a fast serial analog output—or can be converted into digital signals by a fast AD converter integrated on or directly alongside the sensor chip.
The disadvantage here is that the bandwidth of these preamplifiers downstream of the multiplexer has to be very high in order to transmit the sensor signals of many successively sampled thermopile pixels still with image frequencies of from a plurality of Hz to tens of Hz.
This necessitates at least m×n times the frame rate in the case of one preamplifier per array, or m times the frame rate for an array having m columns and respectively one preamplifier per column. With the root of the (noise) bandwidth, however, at the same time the noise of the system also increases and the temperature resolution NETD deteriorates.
High-gain amplifiers which operate stably and accurately require a plurality of amplifier stages, need a relatively large amount of space on the sensor chip and additionally have a considerable current consumption with correspondingly high waste heat, which in turn further reduces the achievable signal voltage of the thermopile elements.
For this reason, it is not possible to accommodate many of such large amplifiers alongside the pixels on the restricted space of a sensor chip.